Refrigerated containers are typically used to transport perishable products. Ideally, the refrigerated containers are intended to maintain a substantially uniform and constant temperature throughout the interior of the container in order to efficiently refrigerated all the products and prevent spoilage. Typically, such uniform constant temperatures are not achieved with the result that there is considerable spoilage of cargo.
The industry uses a range of refrigerated containers, each designed for a specific type of transportation system. For example, containers can be designed for use with trucks and truck trailers (e.g., truck-trailer reefers), sea going ships (e.g., marine reefers), or trains (e.g., rail reefers). These refrigerated containers are typically uniformly shaped rectangular boxes that are sized to be efficiently used with the intended transportation system.
In a typical conventional refrigerated container, a simple refrigerated air distribution system is used that includes a refrigerator and a fan disposed at one end of the container to blow the refrigerated cold air longitudinally into the cargo space of the container. This system is unsatisfactory because cold air does not travel the length of the container and is often blocked by cargo. Thus, cargo further from the fan is often not cooled sufficiently and spoils, while cargo nearer the fan freezes. Because the walls of the container may become warm due to ambient outside conditions, the cargo is sometimes spaced from the walls to reduce heat input and resultant spoilage. This, along with a space between the roof of the container and the cargo for cold air flow, reduces the capacity of the container. Some improvements in this prior art design have been proposed. For example, U.S. Pat. No. 5,187,945 entitled "Refrigerated Container" and issued to David A. Dixon Feb. 23, 1993 ("the Dixon patent") discloses a refrigerated shipping container that uses a multiplicity of fans mounted on sides of the container to blow air transversely into the container to ameliorate the air distribution problem. Aside from the weight penalty imposed by the numerous mechanical parts, the design also introduces more heat into the container from the added electrical equipment.
Another factor in maintaining the desired temperature profile in the refrigerated container also addressed by Dixon is the construction of the container's side walls. In Dixon, the side walls are constructed of a corrugated steel outer wall, with a polystyrene insulating slab cemented to its interior surface. A first plastic sheet is then cemented to the polystyrene slab. A second plastic sheet is attached to the first plastic using metal spacers to create an air duct between the two plastic sheets. Thus, the side walls require several layers of different materials and several fabrication steps, which tend to increase the costs of the side walls. Moreover, in use these layers tend to separate, necessitating frequent costly repairs.
Although Dixon's proposed containers represent an improvement over the previous generation of refrigerated containers, the large number of fans and the complexity of the side wall construction lead to relatively high cost, weight, power consumption and maintenance requirements. Accordingly, there is a need for a simple, efficient, low-cost, refrigerated container that uniformly delivers cold air to all parts of the container. Preferably the container should also be subdividable into zones, each of which could be maintained in a selected temperature range.